1. Field of the Invention
This invention relates to a non-volatile semiconductor memory device (EEPROM), specifically to a data read method of a NAND-type flash memory, in which multiple memory cells are connected in series to constitute a NAND string.
2. Description of the Related Art
A NAND-type flash memory is well known as one of EEPROMs, which is easily achieving a high integration. In a data read mode of the NAND-type flash memory, unselected memory cells in a NAND string are made to operate as pass transistors, and it will be detected the on/off state of a selected cell. That is, the selected cell is applied with a read voltage necessary for judging the cell threshold distribution, and the remaining unselected cells are applied with a read pass voltage that turns on cells without regard to cell data, whereby data is judged in accordance with whether the precharged bit line is discharged or not.
As the integration of the above-described NAND-type flash memory progresses, the reliability of read data becomes problematic. Specifically, the capacitive coupling between adjacent cells influences the read operation. For example, it will be generated such a situation that the floating gate potential in an unselected cell adjacent to a selected cell is not raised sufficiently, and this unselected cell becomes disable to carry a sufficient cell current as a pass transistor. Especially, unselected cells disposed on the lower course side of the selected cell (i.e., on the source line side of the selected cell) are problematic because these are set in a high channel resistance state due to negative feedback serving for lowering the cell current. This cell current decrease at a data read time causes the erroneous read and increasing of the sensing time.
With respect to highly integrated NAND-type flash memories, there have already been some proposals, in which the influences of capacitive coupling between adjacent cells are considered. For example, refer to JP-A-2002-133888. In this application, there is shown that an unselected word line adjacent to a selected word line is applied with a read pass voltage set to be lower than that of the other unselected word lines at a write verify-read time.